Entangled clocks could provide accurate world time standard

Plans for a global network of atomic clocks that are synchronized using quantum entanglement have been unveiled by physicists in the US. The resulting universal time standard would be more accurate than is currently possible with individual atomic clocks, and the network could also be used to do a range of fundamental and applied research, such as mapping the Earth's gravitational field or even testing new theories of gravity. While some of the technologies needed to build the network already exist, other elements still need further development.

Atomic clocks have revolutionized the world in ways that were unimaginable when the first atomic clock was built in 1949. The Global Positioning System (GPS), for example, uses atomic clocks to measure the travel time of signals from four satellites to a GPS receiver almost anywhere on Earth. As clocks have become more accurate, researchers have proposed ever-more-exotic applications such as relativistic geodesy, in which the strength of gravity at various points, and therefore the local density of the Earth, could be mapped by measuring variations in the rates of clocks at different locations. Scientists have also suggested that ultra-accurate atomic clocks placed in outer space could detect gravitational waves.

The ultimate limits on the stability of a single atomic clock are determined by Heisenberg's uncertainty principle, which limits how precisely a system's oscillations can be measured. This has been drastically reduced in recent years with the invention of optical lattice clocks containing thousands of particles, allowing the quantum uncertainty to be reduced by averaging. In the new research, Mikhail Lukin's quantum-optics group at Harvard University has teamed up with Jun Ye's quantum-metrology group at the National Institute for Standards and Technology (NIST), the JILA Lab and the University of Colorado. Ye's team holds the current world record for clock accuracy, and now the two groups have come up with a way to make an even better timekeeper by effectively combining several atomic clocks into one.

Entangled super clock

Their plan involves many clocks, all stationed in different countries and connected by both classical and quantum links. In each cycle of the synchronization process, one clock (the "central clock") sends out one photon from an entangled pair to each of the other clocks, via the quantum link. This allows the clocks to prepare a single, collective entangled state and to transfer this entangled state on to the atoms in the clocks. Each clock then measures this collective entangled state and, after doing so, it uses the classical link to send both its own laser frequency and the phase difference it measures between this laser frequency and the atoms to the central clock. The central clock can then calculate the average phase difference and use it to calculate the correction that needs to be applied to the frequency to bring all the clocks back into phase. The clocks thereby reduce their uncertainty by collectively averaging over all the atoms in all the clocks, effectively behaving as a single, super-accurate atomic clock.

This distributed set-up would provide all the participating parties with access to the ultra-precise time signal at any time, creating a universal time standard. The network would also be more secure against physical attack because, even if one of the systems were disabled, each party could fall back on its own atomic clock. Finally, it might be possible to use changes in the rate of individual clocks to detect gravitational waves or even to test new theories of gravity. "If you have this very good reference, and there is a local node that doesn't follow this reference, then that's a signal," explains Lukin. It could either mean that the clock is not working properly, he says, or it might represent something more fundamental. In principle, one of the clocks could be taken out of the Earth's gravitational field to make non-local tests of fundamental physics, for example, although the engineering challenges would be severe.

Far-fetched but well-reasoned

Eugene Polzik, a quantum-optics expert whose team at the Niels Bohr Institute in Copenhagen built the first atomic clock containing entangled atoms, describes the proposal as "very interesting and important". He says, however, that before the scheme can be realized, two main technical hurdles must be overcome. First, scientists must successfully generate the giant quantum state containing far more atoms than has so far been achieved, and second, quantum repeaters will have to be developed to allow entanglement distribution around the world. "We have to dream," he concludes. "And it's always good to have a far-fetched proposal, which is backed up by reason, and that's exactly what this paper is doing."

Unavoidable gravity decoherence

As a rule the concept of entangled clocks is interesting, but the possible different gravitational fields (hence,the different gravitational time delations) for them at their locations, may lead to their unavoidal mutual decoherence.

this task can be accomplished without entanglement

A world wide clock can be had by simply transmitting a single laser signal world wide and taking the delay time into account on all paths. Based on the conservation of wavefronts the frequency is not changed by gravity potential and since the different clocks are stationary with respect to each other there is no doppler shift. Therefore the frequency transmitted from the master clock is the exact same frequency received at all other stations. Only the time delay is needed which can be readily measured by echoing a signal there and back and seeing the total time for the trip and then taking half that time. Its not necessary to use entanglement or large numbers of atoms to accomplish a super accurate world wide clock system if the conservation of wavefronts is used.

time in gravitational field

The idea is interesting. But there is one small problem. In gravitational field time is non holonomic function (see some details in my paper in arXiv, ePrint gr-qc/0405027 : Reference Frame in General Relativity, 2004, sections 8, 9. This statement means that if you try to synchronize system by close loop, you will observe some difference of time in original point. this means that we can synchronize reference frame only along open way. For Earth difference in time is small but measurable, so we can test it. This is important when we try new theory using such system

Nice idea

Interesting idea, because the frequency of the laser signal, indeed, does not change whatever the value of the gravitation field it has pass through. The precision of the "laser clock" has to depend on its frequency: the highjer the frequency, the higher should be its precision. However, for the delay time between the different laser clocks, the echo signals must follow the same path as the forward signals to avoid any differential gravitational dilation.

A world wide clock can be had by simply transmitting a single laser signal world wide and taking the delay time into account on all paths. Based on the conservation of wavefronts the frequency is not changed by gravity potential and since the different clocks are stationary with respect to each other there is no doppler shift. Therefore the frequency transmitted from the master clock is the exact same frequency received at all other stations. Only the time delay is needed which can be readily measured by echoing a signal there and back and seeing the total time for the trip and then taking half that time. Its not necessary to use entanglement or large numbers of atoms to accomplish a super accurate world wide clock system if the conservation of wavefronts is used.

Really not called for

This type of comment is unpleasant. And, yes, as a matter of fact, a PhD does indicate that a comment may be somehow more valuable ... and worth a little extra consideration and thought, even (or especially) if it sounds a bit odd or strange at first. To me, an advanced degree indicates that a person once studied deeply in a knowledge field and mastered it sufficiently to make a respectable contribution in the eyes of his or her peers. Chances then are very good that that person can do the same thing again in practically any field if sufficiently motivated and interested in doing so. That in itself says a lot about such a person. So give a guy a break for signing his or her name to a comment.